The present invention generally relates to ultrasound imaging. In particular, the present invention relates to ultrasound compound imaging with combined fundamental and harmonic signals.
Ultrasound is sound having a frequency that is higher than a normal person may hear. Ultrasound imaging utilizes ultrasound waves or vibrations in the frequency spectrum above normal human hearing, such as the 2.5-10 MHz range. Ultrasound imaging systems transmit ultrasound into a subject, such as a patient, in short bursts. Echoes are reflected back to the system from the subject. Diagnostic images may be produced from the echoes. Ultrasound imaging techniques are similar to those used in sonar and radar.
A medical ultrasound system forms an image by sequentially acquiring echo signals from ultrasound beams transmitted to an object being imaged. An individual beam is formed by transmitting a focused pulse and receiving the echoes over a continuous range of depths. An amplitude of an echo signal decreases significantly for signal reflectors located deeper in the object due to increased signal attenuation of intervening structures, such as intervening tissue layers. Therefore, a signal-to-noise ratio decreases since noise generated by the ultrasound system's signal amplifiers, for example, may not be reduced to arbitrary low levels.
Forming the best possible image at all times for different anatomies and patient types is important to diagnostic imaging systems. Poor image quality may prevent reliable analysis of the image. For example, a decrease in image contrast quality may yield an unreliable image that is not usable clinically. Additionally, the advent of real-time imaging systems has increased the importance of generating clear, high quality images.
Spatial compounding has become an advanced and important diagnostic tool in a wide range of applications in ultrasound imaging. In spatial compounding, a target is scanned from several angles of insonification or irradiation with sound or other such waves. Multiple received images are then combined or averaged to form a single image. A compounded image typically shows less speckle or interference introduced by scattering which degrades image resolution. A compounded image may also provide better specular reflector delineation than conventional ultrasound images from a single angle. In some ultrasound machines, multi-angle spatial compounding has been implemented on different types of transducers, such as a one-dimensional linear array and a one-dimensional curved linear array.
In current systems producing multi-angle spatial compounding, grating lobes introduce artifacts in a resulting image. Grating lobes are side lobes or secondary ultrasound beams transmitted at angles to a main beam or main lobe. Echoes generated by grating lobe reflections may introduce artifacts in a resulting image.
To maintain a large field of view, multi-angle spatial compounding is currently implemented on non-sector-scan phased-array probes. Non-sector-scan phased-array probes are currently not designed to be steered with big angles as a sector scan phased-array probe. A non-sector-scan phased-array probe has a greater pitch value, such as 1λ-2λ, compared to less than 0.5λ for a sector scan phased-array probe, where λ is a wavelength of a probe center frequency. A first order grating lobe appears at an angle determined by a pitch and a wavelength as follows:GL_ang=180*a sin(sin θ±λ/pitch)/pi, /sin θ±λ/pitch/<1 (in degrees)  (1),where θ is the beam steering angle.
The first order grating lobe appears at an angle between 30 and 90 degrees with pitch at about 1λ-2λ, for example. For example, a probe with pitch=1.5λ has a first order grating lobe angle, GL_ang, of 42 degrees with a grating lobe amplitude 35 dB down from a main lobe when θ=0. When a beam is steered with an angle θ≠0, the amplitude of the grating lobe increases as a steering angle increases. For example, for a steered angle at 30 degrees, the amplitude of the grating lobe increases up to 6 dB down from the main lobe.
Additionally, the grating lobe typically has worse resolution than the main lobe, which results in feather-like artifacts in an image. FIG. 1 shows a main lobe at a 30-degree steering angle with a grating lobe shown at a −9-degree angle that has an amplitude 6 dB down from the main lobe. For some transmit vectors, a grating lobe at one side is included in a field of view, as shown in FIG. 1. The grating lobe generates artifacts, especially when the grating lobe is approximately orthogonal to a specular reflector surface. Artifacts degrade image quality of a spatial compounding image. Artifacts also interfere with a clinician's ability to distinguish tumors and lesions, for example, from normal tissue. Thus, there is a need for a system and method for improved spatial compounding. A system and method that reduce artifacts, such as grating lobes, in a compound ultrasound image would be highly desirable.
One method used to reduce artifacts uses a lower frequency for a bigger steered angle to avoid grating lobes in a field of view as disclosed in “Multi-angle spatial compounding”, Soren K. Jesperen, et. al., Ultrasonic Imaging 20, pp. 81-102, 1998. However, while a lower frequency may improve grating lobe suppression, a lower frequency degrades axial resolution with frequency compounding. Alternatively, a smaller steering angle may be used to reduce a grating lobe level. However, a smaller steering angle suffers from compounding effects, such as speckle suppression and line definition, since less benefit is acquired with a smaller steering angle. Therefore, an improved method and apparatus for reducing or eliminating grating lobes would be highly desirable.